Let the heat plate warm up for about an hour first before placing it in and removing the heat lamp. You'll need to "train" them... meaning, stuff them under it so they get the point. The legs are also adjustable, so you can make the front a little bit taller than the back. Like I said, I keep mine a hair above back height. You'll know you've got the height right when they go under quietly. If they're distress peeping, it's too high. If they won't settle, it's too low.
Honestly, it's good to have both a heat lamp and plate. Lamp can be good for freshly shipped chicks that are chilled, but the heat plate is the way to go long term. Mine wean themselves off around 5 weeks.